1. Field of the Invention
The present invention generally relates to the field of electronic circuitry. More particularly, the present invention relates to the field of crossover network circuitry.
2. Description of the Prior Art
The following six (6) prior art patents were uncovered in the pertinent field of the present invention:
1. U.S. Pat. No. 3,613,022 issued to White et al. on Oct. 12, 1971 for "Branching Circuit for Composite Electrical Signals" (hereafter "the White Patent"); PA0 2. U.S. Pat. No. 3,814,857 issued to Thomasen on Jun. 4, 1974 for "Two-Way Loudspeaker System With Two Tandem-connected High-Range Speakers" (hereafter "the Thomasen Patent"); PA0 3. U.S. Pat. No. 4,100,371 issued to Bayliff on Jul. 11, 1978 for "Loudspeaker System With Phase Difference Compensation" (hereafter "the Bayliff Patent"); PA0 4. U.S. Pat. No. 4,208,548 issued to Orban on Jun. 17, 1980 for "Apparatus And Method For Peak-Limiting Audio Frequency Signals" (hereafter "the '548 Orban Patent"); PA0 5. U.S. Pat. No. 4,495,643 issued to Orban on Jan. 22, 1985 for "Audio Peak Limiter Using Hilbert Transforms " (hereafter "the '643 Orban Patent"); and PA0 6. U.S. Pat. No. 4,525,857 issued to Orban on Jun. 25, 1985 for "Crossover Network" (hereafter "the '857 Orban Patent").
The White Patent discloses a branching circuit for composite electrical signals. The composite electrical signals are connected to a unilateral phase-splitting circuit, a filter network and a differential amplifier. A first output signal from the phase splitter is applied to the input of the filter and to an input of the differential amplifier. The second output from the phase splitter, of opposite phase to the first output is applied to the other input of the differential amplifier. The branching circuit provides a first branched signal at the output terminals of the filter for signal components falling within the pass band of the filter and a second branched signal at the output of the differential amplifier for signal components falling within the stop band of the filter.
The Thomasen Patent discloses a two-way loudspeaker system with two tandem-connected high-range speakers. The high-frequency response and directional characteristics of a two-way loudspeaker system are improved by using two specially differential nearly identical high-frequency speakers both identically connected in parallel to a simple passive crossover network.
The Bayliff Patent discloses a loudspeaker system with a phase difference compensation. It comprises a crossover with a discrete active high-pass filter which is also fed to one input of a subtractor in order to derive low-pass characteristics from the high-pass filter. The loudspeaker system covers different but overlapping frequency ranges; a treble range and a bass range are mounted to radiate from co-planar mouths. Phase delay is introduced by the radiator for the lower range which is compensated by an acoustic delay disposed between the radiator of higher optimal frequency range and its mouth. The acoustic delay takes the form of an exponential horn which introduces a delay corresponding to the displacement.
The '548 Orban Patent discloses an apparatus and method for peak limiting audio frequency signals. The apparatus and method are used in systems employing high frequency pre-emphasis to compensate for steep high frequency rolloff in a receiver. The apparatus and method are useful in standard AM broadcasting to maximize loudness without noticeable distortion. The distortion caused by a clipper is determined by subtracting a clipper's output from its input.
The '643 Orban Patent discloses an audio peak limiter using Hilbert transforms. The limiter effectively provides radio frequency clipping of low frequencies and audio frequency clipping of high frequencies, and thereby little or no harmonic distortion occurs for voice whereas harmonic distortion is permitted for high frequency signals.
The '857 Orban Patent discloses a crossover network. It comprises a first shelving filter, a second shelving filter, a first low-pass filter, a second low-pass filter, a phase corrector and a subtracting means. The first shelving filter receives an audio signal. The second shelving filter is coupled to the output of the first shelving filter. The first low-pass filter is coupled to the output of the first shelving filter. The second low-pass filter and the phase corrector are coupled to the output of the second shelving filter. The subtracting means is used for subtracting two signals coupled to the output of the first and second low-pass filters, and the phase corrector. The band limited crossover network is produced with a high frequency band which is present at the output of the subtraction means and a low frequency band which is present at the output of the first low-pass filter.
In the field of audio there are numerous applications for frequency dividing circuits usually referred to as "Band Filters", "Crossover Networks", or simply, "Crossovers". For example, in the field of loudspeakers, crossovers are used to divide the audio frequencies between two or more acoustic drivers (e.g., woofer and tweeter). In these cases, the crossover may be a passive network built into the loudspeaker cabinets or an electronic means used in "bi-amped" or "tri-amped" systems. Signal processors also use crossover band filters in numerous ways. First order crossover filters are easy to construct and tune to the desired crossover frequency. However, first order band rejection is inadequate for many uses. Higher order crossover slopes are therefore sought after which can provide the needed band rejection and still produces a reasonably flat combined frequency response.
The major problem encountered with crossover filters or networks is obtaining a flat combined frequency response. Using conventional filters, it is impossible to obtain crossover responses that contain matching time responses, and the result is a non-flat frequency response near the crossover region. It is customary to offset the crossover filters somewhat to obtain a usable combined frequency response, but the unmatched phase response of the crossover filters can cause misdirection of the radiation pattern of loudspeaker arrays. A great deal of effort has been made to correct time errors in loudspeaker systems, much of it aimed at compensating for crossover problems, with only limited success. Obviously it would be useful to have a crossover filter which gives a relatively high order of crossover slope but which is perfectly timed matched at all frequencies.
An undesirable result of conventional loudspeaker crossovers is that their crossover point is usually found at only about -1 dB of amplitude when the combined frequency response is optimized. This causes amplifier power to be wasted by the two drivers because they are both at high drive levels for a relatively wide bandwidth either side of the crossover frequency. If the time response of the crossover filters were equal throughout the crossover transition, then the crossover point could be made to occur at -7 dB of amplitude and the combined power delivered to the drivers would be equal at all frequencies. Another advantages of obtaining a -6 dB crossover characteristic would be improved loudspeaker protection since less power would be driven to the speakers in the crossover region. It would obviously be desirable to have a crossover filter which yields a flat combined response with a crossover amplitude of -6 dB.
It would further be useful to have a crossover filter which is easy to construct and tune, and can be constructed from commonly available parts at a low cost.
It is therefore desirable to have a very effective design and construction of a phase coherent crossover making use of the known time response relationship between an all-pass filter and a low-pass filter in a manner which derives a high-pass filter output that is phase coherent with the low-pass filter, crosses the low-pass filter's response at -6 dB, and which can be summed with the low-pass filter output to reproduce a perfectly flat frequency response.
It is also desirable to have a very effective design and construction of a phase coherent crossover making use of the known time response relationship between an all-pass filter and a high-pass filter in a manner which derives a low-pass filter output that is phase coherent with the high-pass filter, crosses the high-pass filter's response at -6 dB, and which can be summed with the high-pass filter output to reproduce a perfectly flat a frequency response.